1. Field of the Invention
The present invention relates to a projection screen, a lens sheet suitably used therein, and a method of manufacturing the lens sheet.
2. Description of the Related Art
A projection screen is constructed of sheets that include a lenticular lens sheet, a Fresnel lens sheet, etc. The Fresnel lens is for the purpose of making the light emitted and projected from an image source substantially parallel with rays that are directed toward a viewing person side and thereby making the brilliance of the screen uniform. On the other hand, the lenticular lens is for the purpose of diffusing the light so that an image may be viewed over a wide range on the viewing side. Accordingly, it is conventionally practiced in general to mix light diffusion fine particles into the lenticular lens sheet. In contrast to this, as the Fresnel lens sheet, conventionally, there has been used a transparent sheet having no light diffusion agent particles mixed thereinto. In recent years, however, in order to eliminate image obstructions due to the rise surfaces (the leading surfaces) of the Fresnel lens, it has become generally practiced to mix the light diffusion agent particles into the Fresnel lens sheet as well.
These lenticular lens sheet and Fresnel lens sheet have hitherto been manufactured with methods such as a thermal pressing molding, extrusion-molding, thermal polymerization molding, ionizing radiation curing molding, etc. Among these methods, the ionizing radiation curing molding method having a merit in that the mold configuration is excellent in reproducibility; and the molding process time length is short, especially a UV curing molding method that uses ultraviolet rays as ionizing radiation has now been generally practiced.
The ionizing radiation curing molding method, as illustrated in FIGS. 4A to 4G, is constructed of the following processes. A temperature adjustment process (FIG. 4A) that adjusts a mold 12 for a lens sheet to a temperature suitable for molding the lens. The first resin coating process (FIG. 4B) that coats a liquid ionizing radiation curable resin over the entire surface of the temperature-adjusted mold 12. A second resin coating process (FIG. 4C) that coats the liquid ionizing radiation curable resin onto the position on the pressurization starting side that is located on the mold 12. A base material supply process (FIG. 4D) that covers the mold 12 with a base material sheet 14 transmitting ionizing radiation therethrough from over the ionizing radiation curable resin layer 13. A leveling lamination process (FIG. 4E) that presses the base material sheet 14 and the mold 12 with pressurizing rolls 15a, 15b from the pressurization starting end side towards the pressurizing termination end side and thereby laminating the base material sheet 14 onto the ionizing radiation curable resin layer 13 while leveling the same (13). A resin curing process (FIG. 4F) that radiates ionizing radiation onto the ionizing radiation curable resin layer 13 from over the base sheet 14 and cures the same (13). An exfoliation process (FIG. 4G) that exfoliates the cured ionizing radiation curable resin 13 from the mold 12 jointly with the base sheet 14.
In the leveling lamination process (FIG. 4E) of the ionizing radiation curing molding method, the light diffusion agent particles that have protruded from the surface of the base sheet repeatedly contact with the forward end portions of the mold 12; and cause damages to the forward end of the mold. This shortens the service life of the expensive mold 12. That method had these problems. They have recently become more and more prominent. The reason for this is that, as the thickness of the lens sheet becomes gradually smaller owing to a demand for the increase in the fineness of the images, etc., the same amount of light diffusion agent particles is mixed into a smaller thickness of sheet. In addition, there has been a demand for the improvement in the optical function of the light diffusion agent particles.
Thereupon, the present invention has an object to provide a lens sheet that without, when molding the resin, damaging the mold can enhance the optical function of the light diffusion agent particles, a projection screen, and a method of manufacturing the lens sheet.
Hereinafter, the present invention will be explained. It is to be noted that although in order to make easier the understanding of the present invention the reference symbols in the appended drawings will additionally be written by being bracketed this does not impose a limitation upon the present invention by the illustrated embodiments.
The lens sheet in the first aspect of the present invention is a lens sheet having a base material sheet (BS) that has a lens formed on at least one surface thereof using an ionizing radiation curable resin, wherein light diffusion agent particles (D) are mixed into the base material sheet and have amass average particle diameter of 3 to 25 xcexcm. Here, the wording xe2x80x9cmass average particle diameterxe2x80x9d is calculated as follows,
xcexa3nd4/xcexa3nd3
where xe2x80x9cnxe2x80x9d represents the number of the particles; and xe2x80x9cdxe2x80x9d represents the particle diameter.
According to the lens sheet of this aspect, the mass average particle diameter of the light diffusion agent particles has been made small. Therefore, enlarging the interval between the troughs of the lens sheet and the surface of the base material sheet is not needed very much. The amount of ionizing radiation curable resin used can be reduced. In addition, the molding of it becomes also stable. Also, generally, to obtain the same light diffusion at half-maximum angle in case the differences between the light diffusion agent particles and the base material sheet in terms of the refractive index are the same, it is more preferable to use the light diffusion agent particles that is small in the mass average particle diameter than to use the light diffusion agent particles that is large in the mass average particle diameter. This is because it is possible to lessen the adding amount (the weight) of light diffusion agent particles. Therefore the sum of the cross-sectional areas of the light diffusion agent particles, which protrude from the surface of the base material sheet, becomes small. And also because the possibility of damaging the mold becomes small. Accordingly, it is preferable that the mass average particle diameter be 25 xcexcm or less. Provided, however, that in case the mass average particle diameter of the light diffusion agent particles is smaller than 3 xcexcm the scatter angle of the light has dependency upon the wavelength with the result that the lens sheet looks yellowish. Therefore, that range is not preferable.
The lens sheet in the second aspect of the present invention is a lens sheet having a base material sheet that has a lens formed on at least one surface thereof using an ionizing radiation curable resin, wherein light diffusion agent particles are mixed into the base material sheet; and, when d represents the mass average particle diameter of the light diffusion agent particles and h represents the interval between the trough (9) of the lens configuration of the lens and the surface on the lens-formation surface side of the base sheet material, there hold true the inequality:
h greater than d/3
According to the lens sheet of this second aspect, the interval between the trough portion of the lens configuration corresponding to the crest portion of the mold and the surface of the base sheet material has been made large. Therefore, it is not possible that the unevenness of the diffusion material damage the crest portion of the mold. Resultantly, it is possible to prolong the service life of the mold.
The lens sheet in the third aspect of the present invention is a lens sheet having a base material sheet that has a lens formed on at least one surface thereof using an ionizing radiation curable resin, wherein light diffusion agent particles are mixed into the base material sheet and have a mass average particle diameter of 3 to 25 xcexcm and, when d represents the mass average particle diameter of the light diffusion agent particles and h represents the interval between the trough portion of the lens configuration of the lens and the surface on the lens-formation surface side of the base sheet material, there hold true the inequality:
h greater than d/3
The lens sheet that is one in the third aspect is equipped with the features of the lens sheet in the first and second aspects. According to the lens sheet of this aspect, the mass average particle diameter of the light diffusion agent particles has been made small. Therefore, enlarging the interval between the trough portions of the lens sheet and the surface of the base material sheet is not needed very much. The amount of ionizing radiation curable resin used can be reduced. In addition, the molding of it becomes also stable. Also, generally, to obtain the same light diffusion at half-maximum angle in case the differences between the light diffusion agent particles and the base material sheet in terms of the refractive index are the same, it is more preferable to use the light diffusion agent particles that is small in the mass average particle diameter than to use the light diffusion agent particles that is large in the mass average particle diameter. This is because it is possible to lessen the adding amount (the weight) of light diffusion agent particles. Therefore the sum of the cross-sectional areas of the light diffusion agent particles, which protrude from the surface of the base material sheet, becomes small. And also because the possibility of damaging the mold becomes small. Accordingly, the possibility of damaging the mold also becomes small. In addition, the interval between the trough portion of the lens configuration corresponding to the crest portion of the mold and the surface of the base sheet material has been made large. Therefore, it is not possible that the unevenness of the diffusion material damage the crest portion of the mold. Resultantly, it is possible to prolong the service life of the mold.
In the lens sheet of this aspect, the interval between the trough of the lens configuration of the lens and the surface on the lens-formation surface side of the base sheet material may be in the range of from 1 to 300 xcexcm, further more preferably in the range of from 2 to 300 xcexcm.
If such is done like that, since a predetermined amount of interval is provided between the trough portions of the lens sheet and the surface of the base material sheet, even if the light diffusion agent particles each of that is larger in diameter than the mass average particle diameter exist on the surface of the base material sheet, there is no possibility that during the manufacturing process the unevenness of the diffusion material will damage the crest portions of the mold. As a result, it is possible to prolong the service life of the mold. Further, there is also the merit that, when the ionizing radiation curable resin is cured, even if it comes to shrink due to curing, the respective lens elements don""t become separated from each other.
Concretely, at the time when the ionizing radiation curable resin is cured, the ionizing radiation curable resin has its volume decreased 5 to 20% due to the polymerization shrinkage. For this reason, the interval between the upper end portion of the configuration of the mold and the lower-side surface of the base material sheet decreases after polymerization in comparison with that before polymerization. At this time, unless that interval is sufficiently wide, owing to the shrinkage of the ionizing radiation curable resin it happens that the so-called xe2x80x9cdeformation (the formed figure is not same as one to be formed by the mold)xe2x80x9d occurs in the configuration of the lens. Or it also happens that the portions having no ionizing radiation curable resin occur between the lens element and the lens element. Or it happens that at the time of exfoliation the lens element is left to remain on the mold. Accordingly, even after curing, it is preferable that the interval between the upper end portion of the mold configuration and the lower-side surface of the base material sheet be at least 1 xcexcm, preferably 2 xcexcm or more. Also, the reason why this interval is made to be a predetermined, or smaller than predetermined, amount is that the portion corresponding to that interval makes no optical contribution and, therefore, when that portion is made greater in thickness than necessary, the lens sheet as a whole unpreferably becomes thick.
Also, in the above-described aspect, the light diffusion agent particles may be made to be resin beads.
If this is done like that, even if the diffusion beads the particle diameter of that is considerably greater than the mass average particle diameter are mixed to protrude from the surface of the base material sheet, since the material quality of the light diffusion agent particles is softer than that of the mold, no damages are caused to the mold. The service life of the mold can thereby be prolonged.
Also, in the above-described aspect, further, the lens maybe formed on the other surface, as well, of the base material sheet.
If the construction is made like that, it is possible to impart a new optical function to the base material sheet.
Further, in each of the above-described aspects, the base material sheet can also be constructed so that it is formed through extrusion molding.
In case this construction is made like that, manufacturing the base material sheet continuously becomes possible, which contributes to the enhancement in the productivity and the reduction in the manufacturing cost.
Also, another aspect of the present invention is a projection screen equipped with the lens sheet according to any one of the above-described aspects.
In this aspect, it is possible to construct the projection screen with the use of the lens sheet equipped with the above-described features.
Further, another aspect of the present invention is a method of molding a lens sheet, the method of molding a lens sheet including a process of forming a base material sheet by sheet-forming a resin wherein light diffusion agent particles have been kneaded, through the use of an extruder, a process of coating an ionizing radiation curable resin onto a mold the surface of that has formed thereon a configuration inverse to that of the lens configuration, a process of further laminating the base material sheet from over the ionizing radiation curable resin that has been coated on the mold, a process of radiating ionizing radiation onto the ionizing radiation curable resin and curing it to thereby integrate it with the base material sheet, and a process of exfoliating the integrated ionizing radiation curable resin with the base material sheet from the mold, wherein, when d represents the mass average particle diameter of the light diffusion agent particles and h represents the interval between the forward end of the lens-formation surface of the mold and the surface on the lens-molding surface side of the base material sheet, the base material sheet is laminated further from over the ionizing radiation curable resin that has been coated on the mold while maintaining the inequality of h greater than d/3.
According to this method of molding the lens sheet, since the interval between the forward end of the lens-formation surface of the mold and the surface on the lens-molding surface side of the base material sheet has been made large with respect to the mass average particle diameter of the light diffusion agent particles, it does not happen that the unevenness of the diffusion material damage the crest portion of the mold and this enables the prolongation of the service life of the mold.
The above-described functions and advantages of the present invention will become apparent from the embodiments that will be explained next.